The present inventors have previously found a novel lyase of microorganism origin which catalyzes a reaction of converting fumaric acid and ethylenediamine into [S,S]-ethylenediamine-N,N'-disuccinic acid (hereinafter abbreviated as SS-EDDS) and the like (said lyase is hereinafter designated as ethylenediamine-N,N'-disuccinic acid ethylenediamine lyase and abbreviated as EDDS-ase) and proposed an efficient process for producing optically active aminopolycarboxylic acids from fumaric acid and various amines which makes use of the catalytic action of the enzyme with various microorganisms (cf. JP-A-9-140390).
However, it has been revealed that although the reaction of producing optically active aminopolycarboxylic acids, such as SS-EDDS, by EDDS-ase is accompanied by only a very slight degree of side reactions, it is an equilibrium reaction and hence leaves a substantial part of starting materials unreacted. Thereupon, the present inventors have found that by making polyvalent metal ions present in the reaction mixture, the reaction equilibrium can be shifted to the product side nearly to complete the reaction and resultantly a marked improvement of the yield can be attained (cf. JP-A-10-52292 and JP-A-10-271999).
The present inventors have further confirmed that though the SS-EDDS formed, in the reaction mixture after nearly completion of the above-mentioned reaction (the reaction mixture is hereinafter referred to as reaction product mixture), is dissolved by forming a complex with the co-existing polyvalent metal ion, it can be easily recovered by crystallization using a mineral acid in the form of crystals of SS-EDDS containing no metal ions. However, the acid-precipitated SS-EDDS is difficult to be soluble in water and, in order to make it easily soluble to widen its field of application, it is necessary to convert it into a monovalent metal salt. Such monovalent metal salts can be obtained by adding an alkali, such as sodium hydroxide, to the SS-EDDS recovered by acid precipitation after reaction, but this method requires an increased number of process steps and hence is complicated.
Moreover, even when the metal compound is recovered by acid precipitation as the salt of the mineral acid, a neutralization with an alkali is necessary for reusing it thus obtained, and resultantly it comes to have a high salt concentration and a problem occurs that the salt unfavorably accumulates successively as the recycle is repeated. Furthermore, it has been confirmed that when the high concentration solution is added as a metal ion source to a reaction mixture comprising fumaric acid, ethylenediamine, alkali, etc., a precipitate which is assumed to be a fumaric acid salt is apt to be formed. Thus, it is difficult that this method can be regarded as a practical means for reusing the metal salt.
Moreover, in order to shift the above-mentioned reaction equilibrium sufficiently to the product side and thereby to obtain a satisfactory yield, it is necessary to use a metal compound of nearly equal mole to the SS-EDDS formed. Such a metal compound, however, ordinarily contains much insoluble matters other than the metal compound. Therefore, in using the compound in practice, it is necessary to use it after purification or to use an expensive purified product.
Accordingly, the object of the present invention is to efficiently produce alkali metal salts of SS-EDDS, taking into consideration the reaction velocity, yield, operation, cost and other factors.